Knowledge of T2* relaxation times at ultra-high field strength is needed for optimizing acquisition parameters and understanding relaxation mechanisms. Standard T2* measurements, such as conventional multi-echo gradient echo (“GE”) sequences, are affected by static magnetic field (B0) inhomogeneity, which is particularly severe at ultra-high field strength, resulting in erroneous T2* values.
The Gradient Echo Slice Excitation Profile Imaging (“GESEPI”) method was developed for T2*-weighted imaging with B0 inhomogeneity compensation. This method recovers signal loss due to intravoxel dephasing in the slice direction, where the voxel size in two-dimensional (2D) imaging is typically the largest, and consequently the B0 inhomogeneity within a voxel is most severe. More specifically, the GESEPI method is based on a conventional 2D GE sequence, with an additional compensation gradient Gc superimposed on the slice rephaser gradient. A series of N GE images are successively acquired using N equidistant Gc values with an increment ΔGc and a range±Gc,max. A three-dimensional (3D) Fourier transform is then applied to the 3D data set to reconstruct a series of N images representing the slice profile at each pixel. Finally, a subset of these images are summed to yield a final corrected image with a slice thickness equal to that of an equivalent conventional 2D GE image.
The mGESEPI method is a multi-echo version of the GESEPI method, where a train of M GE images are acquired at different echo times (TEs) for each compensation gradient Gc. For each TE, a corrected image is reconstructed as in the GESEPI method, and a T2* map is computed by fitting a monoexponential decay pixel-by-pixel to these M corrected images.
The susceptibility-induced gradient in the slice direction Gz,susc that can be compensated for by a given Gc at a time TE is given by the following equation:∫Gc(t)dt=Gz,suscTE  (1)
Since the left-hand side of Equation (1) is constant, the susceptibility-induced gradient Gz,susc that can be compensated for by this method decreases as TE increases. This implies that the T2* measurements are accurate only if the largest Gc value (Gc,max) is able to compensate for the largest Gz,susc (Gz,susc,max) at the last echo. However, satisfying this condition at ultra-high field strength requires a large Gc,max and a large number of compensation gradients N, resulting in excessive acquisition times for in vivo studies.
This mGESEPI method has been used to measure T2* values in the human brain at 3 tesla (T) and 7 T, and in the mouse brain at 14 T. When implemented on the 8 T human whole-body MRI system, this sequence was found to produce accurate T2* measurements, but acquisition times became prohibitive for in vivo studies. Accordingly, a need exists for a method and apparatus which provides faster and accurate T2* measurements.